Extended Abstract (120K)P1.3
Effects of turbulence-induced light fluctuations on photosynthesis in alfalfa
Gengsheng Zhang, Utah State University, Logan, UT; and L. E. Hipps
The effects of turbulence-induced fluctuations of photosynthetically active radiation in alfalfa on photosynthesis in canopy scale are investigated in this study.
Measurements were done at an alfalfa field at Richmond in Utah during the summer in 2000 and 2001. PAR intensities were observed at three heights in the canopy, with 17 point quantum sensors at upper level, 17 sensors at middle level, and 12 sensors at lower level. Incident total and diffuse PAR were also monitored above the canopy. CO2 flux as well as sensible and latent heat fluxes over the canopy were measured using eddy-covariance method. All sensors were sampled at 10 or 20 Hz. Meanwhile, net radiance and soil heat flux were measured to check energy closure. In addition, soil CO2 efflux was measured with LI-6400 portable photosynthesis system.
Properties of sunflecks have been determined. Probability distribution of light was simulated with beta function. As wind increases, difference between the values of parameter a and b in beta function decreases, and frequency histograms tend to be U-shaped. Power spectral analyses show that prominent spectral peak ranged from 0.8 to 4 Hz for the PAR variation in the canopy at LAI of 1.6. Penetration of PAR into canopy is simulated and G-function, which is the fraction of foliage area projected in the direction of the Sun, is recovered. Apart from solar elevation angle and solar azimuth angle, the relationship of wind to G-function is examined.
30-minute average sensible and latent heat and CO2 fluxes are calculated, with wind coordinate rotations, Massman (2000, 2001) correction, and Webb et al (1980) correction. The results show that energy closure is about 0.8. Further correction is done with the assumption that the Bowen ratio was correctly measured, and the energy closure is forced to 1. Over the canopy during the days with clear skies, the sensible heat flux is upwards in the morning and downwards in the afternoon. This indicates that there was advection in the afternoons. CO2 flux is up to about 38 µmol m-2 s-1, mainly depending on the incident PAR level, and having an increase trend as wind increases.
Since wind causes changes in penetration of mean light as well as sunflecks, a comprehensive model is required to interpret the results. The model CUPID is employed to estimate the canopy photosynthesis that would occur under steady-state light with the same average value of fluctuating light. Comparing these values with the measured CO2 flux above the canopy and CO2 efflux from soil indicates the effect of fluctuations in light had on canopy photosynthesis.
Poster Session 1, Agricultural and Forest Meteorology
Wednesday, 22 May 2002, 3:30 PM-3:30 PM
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